1. Technical Field
This invention relates to dispersive optical elements, and particularly to a dispersive optical element having characteristics of both prisms and a gratings.
2. Discussion
Dispersive optical elements have many important uses. Such elements disperse light by deviating the path of light passing through them by an amount that varies with wavelength. Prisms and gratings are the two most widely used dispersive optical elements. Prisms disperse light because their geometry causes light of different wavelengths passing through them to be separated and deviated by different amounts. In diffraction gratings light passing through the grating is diffracted into a series of orders caused by the interference of wavefronts emitted from each slit in the grating. One important usage of dispersive optical elements is the production of spectrometers. Spectrometers have important applications in many fields such as medicine, material sciences, chemistry, and environmental sciences. Spectrometers use dispersive optical elements such as gratings or prisms to permit the analysis of the spectral composition of sampled light.
Grating spectrometers and prism spectrometers each have various advantages and disadvantages. For example, the resolving power of a prism has large variations over wide spectral bands. This is because a prism's resolving power is a function of the prism material dispersion, which varies in a highly non-linear fashion with wavelengths. In most cases, the resolving power for a prism does not match the desired spectral characteristic curve regardless of the choice of materials. For example, where wide spectral coverage is desired the wide variation in the resolving power of a prism spectrometer over this wide spectral band severally limits its usefulness.
Spectrometers utilizing gratings have other disadvantages. In a grating spectrometer the grating can typically be used over only one diffractive order since the optical efficiency in other orders will be very low. This limits the flexibility of the grating. Also, the wide separation of light for different diffractive orders may prevent their detection in a single detector thus limiting their usefulness in many applications. Further, the resolving power of a grating is independent of a wavelength. As a result, the dynamic range of the resolving power is fixed and cannot be adapted to fit a desired spectrometer specification. The dynamic range of the resolving power is the variation in the resolving power with wavelength. For example, it may be desired to have a resolving power of 100 in the wavelength range of 25 to 26 microns, and a resolving power of 1000 in the wavelength range of 5 to 7 microns. In sum, in many applications (such as spectrometers), it is desirable to have a dispersive optical element with a predefined resolving power characteristic curve. It may be desired that the curve be linear, constant or some other defined curve. Whether one chooses prisms or gratings, the flexibility to achieve a desire curve is limited.
Thus, it would be desirable to provide a dispersive optical element that is more flexible in its dispersive characteristics than gratings or prisms. Further, it would be desired to provide a dispersive optical element in which the spectral curve of the resolving power can be predetermined with greater specificity than is possible with either gratings or prisms. Further, it would be desirable to provide such a dispersive optical element that is simple and can be produced at relatively low cost.